This invention is directed generally to active antennas and more particularly to an integrated active antenna for multi-carrier applications.
In communications equipment such as cellular and Personal Communications Service (PCS), as well as multi-channel multi-point distribution systems (MMDS) and local multi-point distribution systems (LMDS), it has been conventional to receive and retransmit signals from users or subscribers utilizing antennas mounted at the tops of towers or other structures. Other communications systems such as wireless local loop (WLL), specialized mobile radio (SMR), and wireless local area network (WLAN), have signal transmission infrastructure for receiving and transmitting communications between system users or subscribers which may also utilize various forms of antennas and transceivers.
All of these communications systems require amplification of the signals being transmitted by the antennas. For this purpose, it has heretofore been the practice to use a conventional linear power amplifier system placed at the bottom of the tower or other structure, with relatively long coaxial cables connecting with antenna elements mounted on the tower. The power losses experienced in the cables may necessitate some increases in the power amplification which is typically provided at the ground level infrastructure or base station, thus further increasing the expense per unit or cost per watt.
Output power levels for infrastructure (base station) applications in many of the foregoing communications systems are typically in excess of ten watts, and often up to hundreds of watts, which results in a relatively high effective isotropic power requirement (EIRP). For example, for a typical base station with a twenty-watt power output (at ground level), the power delivered to the antenna, minus cable losses, is around ten wafts. In this case, half of the power has been consumed in cable loss/heat. Such systems require complex linear amplifier components cascaded into high power circuits to achieve the required linearity at the higher output power. Typically, for such high power systems or amplifiers, additional high power combiners must be used.
All of this additional circuitry to achieve linearity of the overall system, which is required for relatively high output systems, results in a relatively high cost per unit/watt.
The present invention proposes placing linear amplifiers in the tower close to the antenna(s) and also, distributing the power across multiple antenna (array) elements, to achieve a lower power level per antenna element and utilize power amplifier technology at a much lower cost level (per unit/per watt).
In accordance with one aspect of the invention, linear (multi-carrier) power amplifiers of relatively low power are utilized. In order to utilize such relatively low power amplifiers, the present invention proposes use of an antenna array in which one relatively low power linear amplifier is utilized in connection with each antenna element of the array to achieve the desired overall output power of the array.
Moreover, the invention proposes installing a linear power amplifier of this type at or near the feed point of each element of a multi-element antenna array. Thus, the output power of the antenna system as a whole may be multiplied by the number of elements utilized in the array while maintaining linearity.
Furthermore, the present invention does not require relatively expensive high power combiners, since the signals are combined in free space (at the far field) at the remote or terminal location via electromagnetic waves. Thus, the proposed system uses low power combining, avoiding otherwise conventional combining costs. Also, in tower applications, the system of the invention eliminates the power loss problems associated with the relatively long cable which conventionally connects the amplifiers in the base station equipment with the tower-mounted antenna equipment, i.e., by eliminating the usual concerns with power loss in the cable and contributing to a lesser power requirement at the antenna elements. Thus, by placing the amplifiers close to the antenna elements, amplification is accomplished after cable or other transmission line losses usually experienced in such systems. This may further decrease the need for low loss cables, thus further reducing overall system costs.
The use of multi-carrier linear power amplifiers at or near the feed point of each element in the multi-element antenna array improves transmit efficiency, receive sensitivity and reliability for multi-carrier communications systems.